Field of the Invention
The invention relates to a method and a configuration for operating a multistage counter in one counting direction.
Nowadays, an unlimited number of application areas are known in which event counting is to take place. These events may be the frequency of use of a piece of equipment, the passing of persons or vehicles or objects, the registering of a telephone counting clock, the registering of driving power for instance an odometer in a passenger car or an operating time counter of any particular piece of equipment, and last but not least the registered working time or attendance time of an employee at his/her workplace. All these cases are characterized in that they are registered with the maximum possible precision, i.e. that as a rule a high value range of counting values is covered. Furthermore, in the aforesaid cases, there is generally the desire that the counting result cannot be tampered with, e.g. cannot be reset. Such a requirement can be reliably implemented using a single-stage counter that can only count in an incrementing or decrementing fashion from its previous counter reading. This can easily be implemented, for example, by an EEPROM, it then being necessary to provide an EEPROM cell for each counting value, and the EEPROM being capable either only of being written to or only being cleared depending on whether incrementing or decrementing counting is provided.
The first-mentioned requirement, specifically that the value range covered by the counter is to be as large as possible, makes it necessary to provide an EEPROM memory with a corresponding number of memory cells with such a method of implementation. Expressed in numbers, this results in that, for example, in order to obtain a maximum counter reading of 255, precisely 255 counter cells are required. However, nowadays it is now customary to make such configurations as small as possible. The use of a multistage counter with 8 bits, i.e. 8 counter cells also gives rise to a maximum counter reading of 255. However, such a multistage counter (8-bit binary counter) has the disadvantage that when the next counter position is changed the preceding counter position is reset. This leads to a situation in which the implementation of a multistage counter which counts in only one direction and at the same time cannot be tampered with is possible only with great difficulty.
Published, European Patent Application EP 0 321 727 A describes a circuit configuration in which a plurality of EEPROM cells are disposed in a row. Here, a plurality of rows are in turn connected together. The memory cells of one respective row constitute a uniform value level, it being possible to clear the memory contents of a row, by logic monitoring, only if a carry-over into the next highest row has taken place. The configuration disclosed in this publication has precisely the previously mentioned disadvantages of susceptibility to tampering since, as a result of the logic circuit being influenced, unidirectional counting is not reliably ensured. A similar configuration, which is only somewhat more complex, is presented in Published, European Patent Application EP 0 618 591 A, an auxiliary memory cell being provided for rewriting for each next highest row, which auxiliary memory cell can be programmed and also cleared again. This configuration is also easy to tamper with because the auxiliary memory cells can both be written to and cleared.
U.S. Pat. No. 5,264,689 discloses a reloadable telephone card that has a credit counter and a debit counter. When the card is reloaded, a maximum counting value is set on the credit counter. Whenever a unit is used up, a memory cell is reprogrammed and the debit counter is incremented. As soon as the counter readings of the debit counter and credit counter are the same, the card is “empty”. In order to reload the card, it is then necessary first to increment the credit counter. In order to prevent misuse, neither counter can be reset. The result of this is that when the maximum value of the counters is reached the card can no longer be used.